Interferometry has been used for over a century to measure the surface topography of objects, typically optical components, and distances and small changes in such distances. With the advent of lasers having long coherence lengths and high brightness, the field has expanded greatly. Interferometric imaging, as depicted by FIG. 1, has been difficult to implement for objects with surfaces with steps or slopes greater than a half wavelength of light per resolution element of the imaging system, because the phase count is lost, and the height of the surface is known only modulo λ/2, where λ is the wavelength of light used for the interferometer.
If a series of interferograms are recorded with different wavelengths λi, the ambiguity in the phase may be resolved, and the heights on the object surface relative to a particular location on the particle surface may be calculated, as is shown in the patents cited below.